1. Field of the Invention
The present invention relates to a semiconductor device using a crystalline semiconductor thin film formed on a substrate having an insulating surface.
Incidentally, in the present specification, any of a thin film transistor (hereinafter referred to as a TFT), a semiconductor circuit, an electrooptical device, and an electronic equipment are included in the category of the semiconductor device. That is, any device capable of functioning by using semiconductor characteristics will be referred to as the semiconductor device.
Thus, the semiconductor device recited in claims of the present application includes not only a single component, such as a thin film transistor, but also a semiconductor circuit or an electrooptical device formed by integrating such single components, and further, an electronic equipment having those as parts.
2. Description of the Related Art
In recent years, attention has been paid to a technique for constructing a thin film transistor (TFT) by using a semiconductor thin film (its thickness is about several tens to several hundreds nm) formed on a substrate having an insulating surface. With respect to the thin film transistor, the development thereof particularly as a switching element for an image display device (for example, a liquid crystal display device: LCD) has been hastened.
For example, in the liquid crystal display device, trials have been made to apply TFTs to any electric circuit, such as a pixel matrix circuit for individually controlling pixel regions arranged in matrix, a driving circuit for controlling the pixel matrix circuit, and a logic circuit (calculation circuit, memory circuit, clock generator, etc.) for processing data signals from the outside.
In the present circumstances, although a TFT using a noncrystalline silicon film (amorphous silicon film) as an active layer is put to practical use, a TFT using a crystalline silicon film (polysilicon film or the like) is necessary for an electric circuit required the performance of further high speed operation, such as a driving circuit or a logic circuit.
Conventionally, high temperature annealing has been required to form a polycrystalline silicon film having high crystallinity. Such a polycrystalline silicon film is generally referred to as high temperature polysilicon. For the purpose of forming the high temperature polysilicon film, it is necessary to prepare a substrate having high heat resistance so that the substrate can withstand a process temperature near 1000° C. For that reason, in the present circumstances, a quartz substrate (according to circumstances, a silicon substrate) is used.
However, the quartz substrate has a high unit cost, so that the quartz substrate has problems of increasing the cost of manufacture, and further, increasing the cost of a product. Thus, in recent years, attention has been paid to a low temperature polysilicon film formed on an inexpensive glass substrate, and the research of the high temperature polysilicon film has been gradually declined.
The coefficient of thermal expansion of the quartz substrate is about 0.48×10−6° C.−1, which is as small as about {fraction (1/10)} of the coefficient of thermal expansion of silicon (about 4.15×10−6° C.−1). That is, stress is apt to occur between the quartz substrate and silicon, and peeling (film peeling) of silicon or the like is apt to occur at a heat treatment.
Moreover, since it is difficult to make the quartz substrate large, the use of a TFT using the high temperature polysilicon is limited to a liquid crystal display device with a size of about 1 to 2 inches in diagonal for a projection type projector or the like. That is, there is a problem that such a TFT can not be used for display devices of the several tens inch class, such as a display for a note-sized personal computer.
As a method of forming a crystalline silicon film on a glass substrate, there are known techniques disclosed in Japanese Patent Unexamined Publication No. Hei. 7-130652 and No. Hei. 8-78329 by the same assignee as the present application. The techniques disclosed in these publications use a catalytic element for facilitating crystallization of an amorphous silicon film, so that the formation of the crystalline silicon film having excellent crystallinity can be made by a heat treatment at about 500 to 600° C. and for about 4 hours.
Particularly, the technique disclosed in Japanese Patent Unexamined Publication No. Hei. 8-78329 makes crystal growth almost parallel to the substrate surface by applying the above technique. The present inventors et al. refer to the formed crystallized region particularly as a horizontal growth region (or lateral growth region).
However, even if a driving circuit is constructed by using such TFTs, the circuit does not still reach the state in which the required performance is completely satisfied. Particularly, in the present circumstances, it is impossible to construct a high speed logic circuit requiring an extremely high speed operation ranging from megahertz to gigahertz by conventional TFTs.
The present inventors have repeated various processes of trial and error to improve crystallinity of a crystalline silicon film (called a polysilicon film) including crystal grain boundaries. A semiamorphous semiconductor (Japanese Patent Unexamined Publication No. Sho. 57-160121), a monodomain semiconductor (Japanese Patent Unexamined Publication No. Hei. 8-139019), and the like can be cited.
The common concept of semiconductor films disclosed in the above publications is to make the crystal grain boundaries substantially harmless. That is, the most important object is to substantially eliminate the crystal grain boundaries to cause the movement of carriers (electrons or holes) to smoothly move.
However, it can be said that even the semiconductor film disclosed in the above publications is insufficient to carry out the high speed operation required by a logic circuit. That is, in order to realize a system-on-panel having a built-in logic circuit, the development of a completely novel material is required.